1. Field of the Invention
The present invention relates to a multi-blade blower, provided with a scroll casing, which is suitable for use in an air conditioning apparatus for an automobile.
2. Description of Related Arts
In a multi-blade blower, a centrifugal blade wheel is provided in a scroll casing and a scroll passageway is created between the blade wheel and the casing, so that an air flow created by a rotation of the blade wheel is received by the scroll passageway. Such a multi-blade blower can produce a high air pressure and a large air flow, while the dimensions of the blower are small. Thus, this type of blower has a wide range of application such as in an air conditioning apparatus for an automobile and others.
In this type of blower, the scroll casing has a throat at a location where the scroll passageway commences between the scroll casing and the wheel, so that a throttle portion is created at the location of the throat, which causes a rapid change to be created in the pressure at the tongue, which causes a large pressure change to be created at the location of the throat due to the fact that free ends of the blades of the wheel pass by the opposite end of the throat during the rotation of the wheel, thereby producing a high level of operational noise.
In order to obviate the above mentioned drawback, the specification of Japanese Patent No. 102,467 discloses a blower where the throat is divided into two sections along the direction of the rotating axis of the blade wheel, which sections are, along the circumferential direction of the wheel, spaced for a length corresponding 1/2 of the blade pitch. In this arrangement in the prior art, pressure waves of different phases of 1/2 pitch are created when the blades of the wheel pass by the separated sections, respectively, so that the pressure waves of different phases to be canceled, thereby reducing the operating noise.
The scroll housing is formed with an axial air inlet of a bell mouth shape of the air and an air outlet which is tangential to the blade wheel. The rotation of the blade wheel causes the air to be sucked from the inlet and causes the sucked air to be discharged radially outwardly via the blades. The air flow radially discharged is collected in the housing and is discharged from the outlet. The prior art construction in '467 patent is, however, defective in its efficiently for decreasing an operating noise. Namely, in the prior art, the circumferentially spaced throat portions are provided so that they are axially divided. As well known, at a greater distance from the air inlet, the higher the speed of the discharged air flow. Thus, the noise levels (magnitude of the noise waves) are different from each of the blades passing by the circumferentially spaced throat portions at different axial positions. Thus, the canceling of the noise waves is not sufficient enough to fully reduced the noise. Namely, the noises generated at the separated sections of the throat are not fully canceled, resulting in an insufficient noise reduction performance.